Lithium-ion (Li-ion) batteries are rechargeable batteries that have become increasingly common in many consumer products because of their high energy-to-weight ratios, high energy density, and slow self-discharge when not in use. The three primary functional components of a Li-ion battery are an electrolyte, a cathode, and an anode. The electrolyte of a Li-ion battery typically includes a nonaqueous solution of a lithium salt, which is able to carry Li ions between the cathode and anode when the battery passes a current through a circuit. In this regard, the cathode of a Li-ion battery has been mainly formed from a layered oxide material (e.g., lithium cobalt oxide) and the anode material selected for many Li-ion batteries has predominantly been graphite. However, in order to produce Li-ion batteries that have a higher capacity and capacity retention over a number of charge/discharge cycles, researchers have begun investigating a variety of other materials for use as components of Li-ion batteries.
Recently, one dimensional nanowire (NW)-based materials have been identified as candidates for Li-ion battery electrodes due to their desirable characteristics, including: a faster charge transport, better conducting pathways, and good strain relaxation [1-3]. Indeed, silicon (Si) NW arrays, as well as NWs of cobalt oxide (CO3O4), iron oxide (Fe2O3), and tin oxide (SnO2) in a bulk powder form have been shown to retain over 75% of their maximum capacity over ten (10) charge/discharge cycles, and thus hold promise as potential materials for Li-ion battery electrodes [5-8]. However, despite the potential ability of these materials to serve as electrodes, the stability of these materials during cycling either remains unknown, or in some cases, is significantly limited (e.g., capacity fading after about 30-50 cycles).
Of the various metal and metal oxide systems that have been used as anode materials for Li-ion batteries, both Sn and SnO2 are interesting because of their concurrent semi-conducting properties and high capacity capabilities (Sn: 994 mAhg−1 and SnO2: 781 mAhg−1), compared to that of graphite (372 mAhg−1) [11-12]. Notwithstanding the certain apparent benefits that are associated with these metal and metal oxide systems though, significant capacity fading with cycling is still a specific problem in these systems, largely due to enormous volume changes that occur during Li alloying and de-alloying, which subsequently leads to metal segregation and crystallographic deformation [13]. For example, in the case of Sn, the metal segregation and crystallographic deformation has been observed to be as high as 259% [14].
In light of the identified capacity fading drawbacks associated with the above-identified materials, there has been a recent interest in further investigating the use of nanowire-based oxide materials to improve the capacity fading characteristics that are associated with many of these anode materials. Recent studies have shown that SnO2 nanowires and heterostructured SnO2/In2O3 nanowires retain a capacity of around 700 mAhg−1 for up to 15 cycles, but the capacity still quickly fades to approximately 300 mAhg−1 after 50 cycles. Similarly, SnO2 nanorods have been investigated as an anode material, but, again, these materials have also shown a capacity that fades to approximately 400 mAhg−1 after 60 cycles.
In any event, and although the above-described studies have indicated that nanoscale tin oxide-based materials may have certain beneficial characteristics that allow them to be used as anode materials in Li-batteries, current research still indicates that these materials exhibit low capacities that range from approximately 300-620 mAhg−1 after only 50 cycles. As such, an anode material for a lithium battery has yet to be provided that not only remains stable over a number of charge/discharge cycles, but that also exhibits high-capacity retention during cycling.